1. Field of the Invention
The present invention relates to an identification level control method and an optical receiver in accordance with the identification level control method. More particularly, the present invention relates to an identification level control method for an optical receiver that converts an optical signal from an optical fiber into an electric signal and reproduces data after amplification of the converted electric signal by a limiter amplifier, and an optical receiver in accordance with the identification level control method.
2. Description of the Related Art
In an optical transmitter to receive an optical signal transmitted from an optical fiber at a high speed, for example, at 10 Gbps (Gigabits per second), an identification level has to be optimally maintained independently of a transmission distance of the optical fiber.
FIG. 1 is a block diagram illustrating an exemplary structure of a conventional optical receiver.
Referring to FIG. 1, an incoming optical signal from an optical fiber is converted into a current signal by a light receiving element 10, and then the converted current signal is converted into a voltage signal by a preamplifier 12. A limiter amplifier 14 amplifies a signal supplied from the preamplifier 12 via a capacitor 13 to an amplitude level identifiable by CDR (Clock & Data Recovery) 16. CDR 16 extracts a synchronization clock from the signal supplied from the limiter amplifier 14. Then, CDR 16 reproduces data by using the synchronization clock, and supplies the clock CLK and the data DATA to the next stage.
In general, a high gain (about 30 dB to 40 dB) differential amplifier is used as the limiter amplifier 14. In this case, however, the optical receiver may have insufficient receiving sensitivity, because an input offset voltage arises due to non-uniformity and temperature characteristics of transistors in the limiter amplifier 14. In order to prevent occurrence of such an input offset voltage, a direct current (DC) feedback circuit 20 is provided to monitor for a forward output and a backward output and compensate for the input offset voltage.
In the DC feedback circuit 20, an average detection circuit 22 detects an average of the forward output and the backward output of the limiter amplifier 14, and an amplifier 24 differentially-amplifies the detected average. The DC feedback circuit 20 controls an input voltage supplied opposite to a main signal input of the limiter amplifier 14 so that these input voltages can become equal. In order to make the voltages of the forward and backward outputs of the limiter amplifier 14 different intentionally, an offset voltage Voff from an offset circuit 26 is added to the average Vav.
Japanese Laid-Open Patent Application No. 60-197051 discloses a technique for detecting a code error of a signal identified for an incoming digital signal by an identification device and controlling an identification determination value of the identification device to make the code error alleviated.
Also, the phases of an input signal and an oscillation output are synchronized to make a difference between the phases fixed. Then, a clock timing corresponding to the transmission rate of the input signal is extracted, and the phase of the detected clock is sequentially swept to a voltage threshold for the input signal in a reproduction control circuit. Furthermore, it is determined whether levels of adjacent monitor points are the same, and data reproduction is controlled by using an identification point, where errors least occur in an eye pattern effective area, as an optimal point.
Ideally, it is desirable that an identification level be maintained at an optimal level of an input waveform, that is, at a BER (Bit Error Rate) minimum level. However, it is not practical to install a large-scale circuit to detect BER into an optical receiver. Also, another method of maintaining an identification level at an optimal level based on a feedback signal from a FEC (Forward Error Correction) circuit is proposed. However, such a method cannot be applied to transmission systems without a FEC circuit.
Accordingly, in a conventional optical receiver, an optimally adjusted identification level is fixed and maintained stably. However, as an optical transmission speed is higher, waveform distortion due to dispersion of the optical fiber becomes more influential.
FIGS. 2A and 2B show exemplary optical waveform before and after optical fiber transmission of 10 Gbps, respectively. As shown in FIGS. 2A and 2B, an optical waveform has some distortion features. The first feature is that a large overshoot occurs. The second feature is that cross points of rising curves and falling curves are positioned downward and the duty cycle becomes small.
The limiter amplifier 14 shown in FIG. 1 has no AGC (Auto Gain Control) function. Accordingly, if an input amplitude is large, an output amplitude of the limiter amplifier 14 is limited. Specifically, the amplification is performed in a condition where the input waveform is partially cut.
If the offset voltage Voff of the offset circuit 26 is zero, the amplifier 24 operates to make an forward output and a backward output of the limiter amplifier 14 equal to the average voltage thereof. Accordingly, in an output of the limiter amplifier 14, an area “a” in the vicinity of cross points of an input waveform-is cut, as illustrated in FIG. 2A. Since a portion of the optical input waveform is cut in the limiter amplifier 14, such a cutting level in the limiter amplifier 14 substantially becomes equivalent to an identification level.
If the dispersion value of an optical fiber is fixed, the identification level can be optimally regulated depending on the dispersion value. In fact, however, the dispersion value is variable depending on transmission distances and kinds of optical fibers. Also, the dispersion value may vary during operation in a system, in which a transmission path changes during operation thereof, such as a recent metro transmission apparatus. For these reasons, it is necessary to widen a range of dispersion value (dispersion tolerance) receivable by an optical receiver as much as possible.
As shown in FIG. 2B, after optical fiber transmission, the cross points are positioned downward to the low level side (optical quench side). In such a case, a cut portion in the limiter amplifier 14 follows the cross points, and the area “b” at the low level side of the input waveform is cut and amplified. Accordingly, although the identification level is set around the center of the waveform before transmission, the identification level is shifted to the low level side after the transmission. This difference deletes a low level side noise margin, resulting in degradation of BER and limitation of the dispersion tolerance.
In actual optical transmission, the high level side (optical emission side) and the low level side have different S/N (Signal-to-Noise) ratios. Specifically, the high level side has a poor S/N ratio in general. Thus, it is necessary to set the optimal value of the identification level at the side slightly lower than the waveform center and adjust the limiter amplifier 14 to cut the side slightly lower than the waveform center. The offset circuit shown in FIG. 1 cuts the slightly lower side by additionally supplying the offset voltage Voff from the offset circuit 26 to the amplifier 24.
Japanese Laid-Open Patent Application No. 2003-018140 discloses a transmitter to address the above-mentioned problem. However, since the disclosed transmitter needs a PLL (Phase-Locked Loop) circuit to extract a clock, it is impossible to prevent a size increase of the circuit.